Abstract: A detailed and comprehensive study was made of the hydrochemical properties of the streams flowing through Swildon's Hole in 1999 and 2000 with additional data being collected in 2002 to permit the inclusion of data from a "normal" dry summer. Measurements were made and water samples were collected in the cave and from the surface streams feeding it. The aim was to measure the concentrations of all ionic species present in the samples in significant levels (accuracy and precision being checked by calculating ion balances). For each sampling trip in the cave it was possible to calculate progressive increments of total hardness and discharge as the stream flowed through the cave. These increments took place as water from the various stream inlets joined the Main Stream. Seasonal changes were assessed in the characteristics of many of the "drip and trickle" inlets. Although many properties showed low variability, data for aggressiveness were particularly instructive. In most inlets beyond Rolling Thunder, pronounced seasonal changes of aggressiveness were found. These changes were consistent with independent observations concerning levels of carbon dioxide in the air in the cave. In addition, all of the inlets beyond Rolling Thunder are contaminated with nitrate, chloride, sodium and potassium, strongly indicating the presence of contamination by human or animal waste. At the six inlets beyond Rolling Thunder as far as Sump I, the presence of faecal bacteria was confirmed. Ion balances were unsatisfactory in most samples that were seriously contaminated by nitrate. Ion balances were satisfactory in the large majority of samples that were not contaminated with large levels of nitrate. Comparisons with earlier data showed that hydrochemical characteristics in the cave have remained stable for 40 years.